1. Field of the Invention
The present invention relates to the provision of a modulated power supply for an amplifier, and particularly but not exclusively to an efficient envelope-tracking modulated power supply delivered to a radio frequency amplifier.
2. Description of the Related Art
It is well-known in the art that the efficiency of radio frequency (RF) power amplifiers can be improved by implementation of envelope tracking techniques. FIG. 1(a) shows a known efficient envelope tracking implementation, generally denoted by reference numeral 100, a detailed description of which can be found in British Patent No. 2398648 in the name of Nujira Limited. A modulated RF input signal on a line 120 is generated from a complex baseband signal provided by a baseband functional block (or baseband system) 102. The modulated RF input signal on line 120 provides an input for an RF power amplifier 106. As illustrated in FIG. 1, and discussed further hereinbelow, the RF input signal on line 120 suffers a path delay as represented by delay stage 107, such that a delayed version of the modulated RF input signal on a line 122, at the output of the delay stage, forms the input to the amplifier 106. The modulated RF input signal on line 122 is amplified by the RF power amplifier 106 to provide an output RF signal on line 130. The amplification of the RF signal provides a signal at a suitable level for driving an RF transmitter antenna 118 connected to the output of the RF power amplifier 106.
The output of the baseband system 102 is also provided as an input to pre-processing circuitry 101. The pre-processing circuitry 101 provides a signal on line 125 as an input to a modulated power supply stage 116, which in accordance with the technique such as described in UK Patent No. 2398648 generates a modulated power supply on line 126 for delivery to the power supply input of the RF power amplifier 106. Preferably the modulated power supply stage 116 includes a coarse signal path 117 for generating a coarse voltage approximation of the desired power supply voltage, and a correction signal path 119 for generating a correction voltage to be combined, in a combiner 121, with the coarse voltage to generate a modulated supply voltage for the amplifier 106 on line 126.
As denoted in FIG. 1(b), the pre-processing circuitry 101 may include a number of functional blocks. An envelope signal is extracted by an envelope detector 108 from the modulated RF signal on line 120 at the output of the baseband system 102. The envelope detector 108 is a suitable means adapted for obtaining the magnitude of a complex signal. The extracted envelope signal is provided from the envelope detector 108 and is applied to a shaping table 110, which may be a look-up table or a non-linear function. The output of the shaping table 110 is provided as an input to a digital-to-analogue converter 112 for conversion from a digital to an analogue signal. The analogue signal is then provided as an input to a filter stage 114 comprising a reconstruction filter. A shaped, filtered signal based on the extracted envelope is then generated on line 125, at the output of the filter 114, and forms the input to the modulated supply 116.
As illustrated in FIG. 1(a), each stage 108 to 114 of the pre-processing circuitry 101 adds a delay, particularly the reconstruction filter 114. There is an overall delay of the pre-processing circuitry denoted as T1, illustrated in FIG. 1(a).
As also illustrated in FIG. 1(a), and mentioned hereinabove, there is a path delay, denoted by block 107, in the path from the baseband system 102 to the input to the RF amplifier 106 on line 122. This is denoted as T2, illustrated in FIG. 1(a).
The modulated supply 116 also introduces a delay, denoted as T3, illustrated in FIG. 1(a).
Thus overall there is a path delay of T2 between the output of the baseband system 102 and the input to the amplifier 106 on line 122; and a path delay of T1+T3 between the output of the baseband system 102 and the input to the supply terminal of the amplifier 106 on line 126.
The timing delay T3 introduced by the modulated supply can be reliably estimated and compensated for. However the timing delays T1 and T2 cannot be reliably estimated, and therefore cannot be reliably compensated for. The difference between the pre-processing delay T1 and the path delay T2 cause a timing mis-alignment—or timing error—between the RF input signal on line 130 at the output of the RF power amplifier, and the modulated power supply voltage on line 126 provided to the RF power amplifier, which is intended to be tracking the RF input signal on line 122. In practice the RF output signal on line 130 and the supply voltage on line 126 are connected at a common node, and the tracking supply voltage must be time-aligned to the generated output voltage on line 130 as best as possible to maximise efficiencies.
The effect of this timing mis-alignment can be understood with reference to FIGS. 2 and 3.
With reference to FIG. 2a, there is illustrated an RF signal designated by reference numeral 202, and an envelope signal extracted therefrom designated by reference numeral 201. In the example of FIG. 2a, the envelope signal is well-aligned with the RF signal. As illustrated in FIG. 2b, this good alignment means that for every instantaneous level on the RF envelope there is a unique gain value, as represented by the curve denoted by reference numeral 203. The same applies to the phase shift, but this is not shown in the figures. When the gain curve is corrected by a suitable simple one-dimensional means, the resulting transmitted spectrum, as denoted by reference numeral 204 in FIG. 2c, is well-controlled.
With reference to FIG. 3a, there is illustrated a scenario where timing mis-alignment occurs between the RF signal and the envelope signal. The RF signal is generally denoted by reference numeral 302, and the corresponding extracted envelope is generally denoted by reference numeral 301. Comparing FIGS. 3a and 2a, it can be seen that in FIG. 3a there is a significant mis-alignment between the RF signal 302 and the envelope signal 301 in FIG. 3a. As illustrated in FIG. 3b a gain curve denoted by reference numeral 303 thus results, which no longer has a unique gain value corresponding to every RF level. If a simple correction means is used, the result is asymmetric and as illustrated in FIG. 3c unwanted sidebands denoted by reference numerals 307 and 308 are generated in addition to the desired spectrum 304, which can potentially interfere with other functions or processes that are occurring in adjacent channels at the same time.
The timing problem occurs purely because of delay. Component tolerances are one cause of delay and result in spreads of delay.
This timing problem is addressed in the prior art by providing means for rectifying the mis-aligned signals of FIG. 3a to be aligned, as best as possible, in accordance with the ideal scenario of FIG. 2a. 
It is known in the art to provide a delay stage, for example in the input path to the amplifier input on line 122 in FIG. 1(a) (not the path delay 107), in order to mitigate for the delay caused in the pre-processing stage 101 as described with reference to FIGS. 3a to 3c. This assumes that the delay caused in the pre-processing stage 101 is greater than that caused by the path delays represented by block 107 in FIG. 1(a). The aim is for the delay stage to re-align the signals as best as possible. Such a delay stage is intended to delay the input signal on line 122 to the RF amplifier on line 122, to attempt to achieve alignment between the modulated supply signal on line 126 (which has been unavoidably delayed by the pre-processing stage 101) and the output signal on line 130.
As noted above, a complication arises in that component tolerances as a result of manufacturing techniques result in spreads of delay. Therefore the delay may not be fixed, and ideally some means for adjusting delay is required, to compensate for production tolerances and temperature, as well as expected delays due to the operation of the circuitry in the pre-processing path. There are techniques known in the art to provide adjustable delays. However there are problems associated with these techniques. In particular there is a requirement to provide a means for measuring the value of the mis-alignment, in order to compensate for the mis-alignment, and achieving this measurement can be complex.
Known methods include detecting the RF output signal from the RF amplifier, i.e. the signal on line 130, and demodulating it at baseband or RF, and then looking for loops in the amplitude-modulation (AM) and phase-modulation (PM) correlation plots. However at baseband this adds considerable complexity, and would therefore not be suitable for implementation in, for example, mobile equipment such as mobile telephone-equipped handsets.
In an alternative known method the RF signals may be demodulated at RF. However with multiple-band equipment, such as multiple-band telephony-equipped mobile equipment, this would complicate the transceiver architecture.
When the envelope signal is a shaped version of the RF magnitude signal rather than the actual RF magnitude signal, a problem arises in using the RF signal to determine timing alignment measurement. In a modulator arrangement that processes the envelope in the digital domain, there may be no access to the unshaped RF magnitude signal. Using cross-correlation with a shaped envelope signal would result in maxima when the signals are time aligned, but these maxima are not sensitive to small timing errors, and there is no direction information.
It is therefore an aim of the present invention to provide a technique which addresses one or more of the above problems. In particular it is an aim of the invention to provide a technique for efficiently measuring or determining the timing misalignment between an RF signal to be amplified and an envelope-tracking modulated power supply signal to be delivered to the amplification stage, to allow for compensation for such misalignment.